I recall someone spotting cracks on their white iPhone the moment he took it out of the box at home. When he went back to the store they accused him of dropping it and wouldn't exchange it or something.
I just noticed it today, with this thread. My white one has a barely noticeble crack on the bottom. Never dropped - damn, never even went out with it, I'm still waiting for the damn invisible shield to come.
Pretty sure that they even said the back is plastic during the keynote about it.